1. Field of the Invention
The present invention relates generally to a signalling system between nodes such as exchanges or transit switching equipments and, more particularly, to an inter-node controlling method and apparatus in a digital network constructed of, e.g., a digital link.
2. Description of the Related Art
In recent years, utilization of a communications technology of ISDN (integrated services digital network) has been developed. In Japan, the TTC (Telecommunication Technology Committee) recommends the common-channel signalling as a standard system of common-circuit signalling between circuit exchanges in a private network in conformity with the international standards such as CCITT (comite consultatif international telegraphique et telephonique).
With this recommendation, there is promoted an attempt to offer products of ISDN-PBX (private branch exchange) in which standard or maker's own non-standard common-circuit signalling is applied to a circuit exchange of the private network. Developed are a variety of communications services utilizing characteristics of such common-circuit signalling and exhibiting a high added value.
On the other hand, a high-speed digital circuit of the public network is hired as a transit line or trunk between circuit exchanges) (hereinafter simply called "exchanges" in the digital network. The network is constructed by time-division per time slot on the order of 64 Kbps. A charge for use of such a circuit channel is generally of a monthly fixed rate system and comparatively high.
In the common-circuit signalling such as the above common-channel signalling, control signals of call initiating, dialing and connecting can be treated as a message. It is therefore possible to transmit and receive a large amount of and multiple pieces of control information between the exchanges.
For instance, according to a conventional network system using the common-channel signalling, the control information per service circuit, i.e., service channel is collected for a plurality of channels. The thus collected control information is transmitted and received via a private channel between the exchanges.
FIG. 1 illustrates one example of the conventional network system using the common-channel signalling.
The system of FIG. 1 includes exchanges 101, 102, a digital circuit 103 and a plurality of terminals 108, 109.
The exchanges 101, 102 each have calling processors 104, 105 and signalling processors 106, 107. The calling processors 104, 105 perform a calling process relative to the exchange. The signalling processors 106, 107 are functional elements for transmitting and receiving the control information of the common-channel signalling. A plurality of terminals (telephones, etc.) 108, 109 are connected to each of the exchanges 101, 102.
The digital circuit 103 is a high-speed digital communications circuit and includes communications channels a, b, c, d. These communications channels a, b, c, d each include a communications band on the order of, e.g., 64 Kbps. The communications channels a, b, c constitute service channels. Then, the communications channel d constitutes a private channel, i.e., private control channel for transmitting and receiving the control information based on the common-channel signalling.
Referring to FIG. 1, the calling processors 104, 105 of the exchanges 101, 102 each transmit and receive the control information on the service channels a, b, c and the control information on an inter-exchange operational management maintenance between the signalling processors 106, 107. The signalling processors 106, 107 each transmit and receive the control information received from the calling processors 104, 105 between the signalling processors 106, 107 of the corresponding exchanges by use of the private control channel d.
As explained above, one or more control channels d shown in FIG. 1 are needed for the common-channel signalling in the conventional system.
In this connection, FIG. 2 illustrates a relationship between a transmission rate and a communications channel number in the high-speed digital circuit available for domestic use in Japan by way of one example of the high-speed digital circuit of the public network that is employed in the above-mentioned network system. FIG. 2 shows the relationship of channel number versus the one-circuit transmission rate in a transmission path when effecting, e.g., an A/D (analog-to-digital) conversion of a voice into a digital signal and transmitting the signal at 64 Kbps.
More specifically, in the case of a circuit of 64 Kbps, only one 64 Kbps communications channel can be taken. In the case of a circuit of 128 Kbps, two 64 Kbps communications channels can be taken. In the case of a circuit of 256 Kbps, four 64 Kbps communications channels can be taken. Similarly, in the case of a circuit of 1.5 Mbps, twenty four 64 Kbps communications channels can be taken. The digital circuit illustrated in FIG. 1 corresponds to the circuit of 256 Kbps. There are employed four 64 Kbps communications channels a, b, c, d.
FIG. 3 is a connection chart of exchange-to-exchange signalling.
To be more specific, a piece of call setting information is transmitted from a call outgoing exchange to a call incoming exchange in response to a call initiating request given from a terminal connected to the call outgoing exchange. When the call is received on the incoming side, the incoming side is brought into a call incoming accepted status. The call setting acceptance information is transmitted to the outgoing side which also comes into a call initiating acceptance status. Next, calling information is sent from the incoming side, and the incoming side is put into an in-call status. Then, the outgoing side is informed of calling. Further, when the incoming side sends a response notification and becomes a response status. Then, the outgoing side is brought into an in-communications (active) status and sends a piece of response confirming information. The incoming side is also thereby put into the active status. The communications are thus conducted. Disconnect information is sent from the outgoing side in response to a disconnect request. This disconnect information is received by the incoming side and informed of the disconnection. Hereat, the incoming side issues release information and makes a release request. With this operation, the outgoing side becomes a null status, and, simultaneously, release complete information is transmitted to the incoming side. The incoming side is, when receiving this information, brought into the null status, thus completing the communications.
In the conventional system, respective items of control information as shown in FIG. 3 are transferred and received via a control channel for exclusive use of control.
According to the conventional network system which adopts the common-channel signalling, one or more dedicated control channels have to be certainly prepared for transmitting and receiving the control information on the service channel communications except the communications channels as service channels based on a calculation from a traffic.
The control information for the plurality of service channels is transmitted and received between the exchanges via the above-mentioned dedicated control channels. Hence, a channel active efficiency decreases with a fewer number of service channels. Namely, the following relationship is established:
10-Channel Control Information Quantity.div.Carriable-via-One-Channel Information Quantity&gt;&gt;2-Channel Control Information.div.Carriable-via-One-Channel Information Quantity
Accordingly, in the conventional system based on the common-channel signalling, if the service channel number calculated from the traffic is as small as several channels or less, the active efficiency of the dedicated control channel is small and therefore not economical.
Further, seeing from a different angle, as far as the communications services offered according to the common-channel signalling are not sufficiently useful to the customers, they apt to think it expensive to prepare the private channels. The customers are conservative about adopting the common-channel signalling, and this in turn hinders a spread thereof.
As discussed above, the increment in charges for using the circuit due to the adoption of the common-channel signalling is one of factors to hinder the spread of the common-channel signalling.